When the joint function of an elbow joint is lost due to rheumatism, rheumatoid arthritis, external injury or the like, replacement with an artificial elbow joint is performed in the interest of greater quality of life. In such an artificial elbow joint, conventionally, stems of the artificial elbow joint are inserted in the humeral and in the ulna and coupled together with a bendable mechanism in a separable manner as seen in Patent Literature 1. In this artificial elbow joint, the two stems must be inserted at the same time into the humeral and ulna. For this reason, the surgery is difficult; and since no rotational function is involved, the artificial elbow joint is functionally inferior to the biological joint.
In view of the above, such an artificial elbow joint as seen in Patent Literature 2 is proposed. In this artificial elbow joint, a substantially cylindrical trochlea and a joint surface member that has a concave curved surface for rotatably receiving the trochlea are prepared separately, and the stems of these elements are inserted into the humeral and the ulna, respectively. In the humeral side component of this prior example, the stem is made straight in shape and is inserted inside the humeral. However, the anatomical shape of the humeral is made so that the diaphyseal part at the distal end is hollowed out on the lower surface side (which is the lower surface side when the palm is extended face up) in what is called the olecranon fossa, and the entirety of the humeral curves (protrudes) gently downward (such a shape is called a “lordotic shape”). As a result, the stem of the humeral side component becomes exposed at the olecranon fossa, and sufficient support strength cannot be obtained. For that reason, what has been done is to form a concavity inside the trochlea which is at the base of the stem, so that the distal end of the humeral is sandwiched by the concavity to supplement the support strength. However, in this structure, the bone must be cut to make the distal end to conform to the shape of the concavity, adding difficulty to the surgery, and placing a great deal of burden on the patient.
Furthermore, the joint surface of the ulna side component and the stem extending from the joint surface are all made of a medical resins (a super-macromolecular polyethylene), and the stem is inserted in the ulna and secured by cement. However, these components are low in strength due to the properties of the material and inferior in terms of support strength, and they also would cause micromotion (which is repetition of minute motions when a force is applied and the component itself deforms), damaging the bone in the periphery of the stem. As a result, because both the humeral component and the ulnar component have weak support strength, loosening occurs in the course of use, requiring re-surgeries or the like.
Patent Literature 1: Japanese Patent Application Laid-Open No. 54 (1979)-132391
Patent Literature 2: Japanese Patent Application Laid-Open No. 2000-107209